1. Field of the Invention
The invention relates to a method for identifying an incorrect position of a semiconductor wafer during a thermal treatment in a process chamber which is heated by means of infrared emitters and is transmissive to infrared radiation, wherein the semiconductor wafer lies in a circular pocket of a rotating susceptor and is held at a predetermined temperature with the aid of the infrared emitters and a control system, and wherein thermal radiation is measured by means of a pyrometer, an amplitude of the fluctuations of the measurement signal is determined and an incorrect position of the semiconductor wafer is assumed if the amplitude exceeds a predetermined maximum value.
2. Background Art
A thermal treatment is used for example for the rapid heat treatment of a semiconductor wafer at high temperature (“rapid thermal anneal”, RTA, or generally “rapid thermal processing”, RTP) or for chemical vapor deposition (CVD), in particular for the deposition of an epitaxial layer on a polished semiconductor wafer.
In this case, the semiconductor wafer generally bears on a support referred to as a susceptor. The susceptor has an edge region and also a pocket lying within the edge region. The depth of the pocket generally corresponds approximately to the thickness of the semiconductor wafer to be treated. During the deposition, the susceptor rotates, and therefore so does the semiconductor wafer lying on it, in order to ensure that the entire wafer surface is treated as homogeneously as possible.
Care must be taken to ensure that the semiconductor wafer lies completely in the pocket of the susceptor during the deposition of an epitaxial layer. A number of possibilities for ascertaining whether this is actually the case is known in the prior art. By way of example, U.S. Pat. No. 6,217,212B1 describes that the signal of a pyrometer directed onto the surface of the semiconductor wafer can be used for this purpose. If the semiconductor wafer is not situated completely in the pocket, then the pyrometer, on account of the vertical wobble and the angular change of the inclined rotating semiconductor wafer, detects a signal oscillating with a relatively high amplitude, the frequency of which corresponds to the rotational frequency of the susceptor. In the case of a semiconductor wafer lying completely in the pocket, by contrast, the amplitude is significantly smaller.
The pocket is generally a few millimeters larger than the semiconductor wafer. WO02/065510A1 teaches that a centered positioning of the semiconductor wafer in the pocket of the susceptor is also important in order to avoid temperature inhomogeneities on the semiconductor wafer. Spacers are offered as a solution, which are intended to ensure that the semiconductor wafer is situated in a defined, centered position in the pocket. This also prevents the semiconductor wafer from floating on the susceptor after placement of the wafer. However, the spacers lead to local turbulences of the process gas, which result in a locally nonuniform thickness of the epitaxial layer in the regions of the semiconductor wafer which adjoin the spacers.
An alternative solution provides for tracking the semiconductor wafer, held by a robot arm by means of an edge gripper, during its transport into the reactor by means of a camera and for comparing its actual position with a predetermined desired position at specific points in time. On the basis of the results of this comparison, the subsequent robot movements are adapted in order to compensate for the deviations ascertained. This is described in US2003/0231950A1. However, this solution has the disadvantage that it exclusively ensures the correct placement of the semiconductor wafer on the generally three pins projecting through the susceptor. When the pins are lowered and the semiconductor wafer is laid onto the susceptor, however, the wafer can still change its position, for example as a result of floating on a gas cushion between susceptor and semiconductor wafer or as a result of vibrations caused by the pins as they are lowered. Incorrect positions of the semiconductor wafer which are attributable to such causes cannot be ascertained in accordance with US2003/0231950A1.